Variable capacitance semiconductor transducers with built-in reference capacitors have been constructed and used, for example, to sense pressure variations, acceleration forces and the like. Parallel plate capacitors are known wherein one plate is formed of a semiconductor material and the other plate is a dielectric, such as quartz. Generally, oppositely disposed surface areas of the respective plates are metalized to provide the conductive regions of the capacitor. Doping the semiconductor plate with a high impurity concentration has also been utilized to form a conductive region in the semiconductor capacitor plate. Typically, the semiconductor plate includes a thin diaphragm portion which deflects in response to the applied force, such as a pressure differential across it.
The accuracy of such semiconductor pressure transducers, especially in the microbar pressure range, is adversely effected by changes in capacitance due to thermal effects and other environmental conditions acting on the transducer additional to the force to be measured. Temperature changes cause variation in plate separation due to thermal expansion or contraction of the material acting as a spacer between the plates. Plate area is also varied by thermal expansion and contraction of the plates. These variations in plate separation and area are sometimes called thermal offset. Temperature changes may also cause deflection of the sensor diaphragm due to creation of stress within the diaphragm parallel to the surface of the plates.
Correction of these thermal effects, for example, by calibration or selection of dielectric material having a coefficient of thermal expansion similar to that of the semiconductor material, is expensive and time consuming and often not entirely effective. The accuracy and utility of a capacitive transducer can be enhanced if the thermal effects are distinguished from the effects of the pressure or other force being measured. A variable capacitance transducer is shown in U.S. Pat. No. 4,420,790 to Golke et al for measuring pressure variations, having an integrated semiconductor reference capacitance transducer. An epitaxial layer on the surface of a silicon substrate wafer forms a diaphragm over an aperture through the silicon wafer. A second similar area of the epitaxial layer is positioned over an area of the silicon substrate wafer having no such aperture. Thus, the second, reference transducer is not responsive to environmental pressure. By comparing the capacitance of the two transducers, the thermal and other effects which deform the reference transducer can be distinguished from the effects of the environmental pressure deforming the primary transducer.
The Golke patent suggests that in prior semiconductor transducers, thermal stress was almost totally due to the use of an upper plate of dielectric material having a different coefficient of thermal expansion from that of the lower semiconductor plate. The Golke device employs silicon wafers for both the upper and lower plate. The principle and reference transducers of the Golke device have a common upper plate of monocrystalline silicon doped with N+impurities to provide electrical conductivity. The gap between the upper plate and the two lower plates of the transducers is determined by the aggregate thickness of a number of stacked films plus eight polysilicon stops. The polysilicon stops are secured to a passivation layer on the upper plate by a thin film technique, such as vacuum deposition. Golke notes that the stops are preferably all of the same height to ensure an equal separation between the plates and the two transducers. There is a recognized need to ensure the uniformity of the spacing between the plates of transducer devices of this type, having reference transducers integrated with a primary transducer.